I understand that it’s possible to rejuvenate old lilacs by hard pruning them to about 8″ from the ground. I read that the best time to do this is March/April – before the growing season begins. I’m sure this will produce green shoots but no buds for blooms. I just don’t want to kill the plant. What do you suggest?
Many thanks in advance!
Thank you for contacting the Toronto Master Gardeners regarding your overgrown lilacs.
Lilacs flower on old wood, and should be pruned when the lilac blooms begin to fade. Lilac shrubs should be pruned regularly every year, to promote vigorous growth, a good framework of stems and to enhance flowering.
There are two approaches to rejuvenate a mature lilac. The less drastic approach is called renewal pruning and involves removing one third of the older stems all the way to the ground each year for three consecutive years. After three years of pruning this way, new shoots should comprise the bulk of the plant. The plant will begin to bloom all over, and you can do regular maintenance trimming from that point on. The best time to do rejuvenation pruning is early in the spring before the shrub leafs out.
If, on the other hand, you cannot stand the look of your shrub, and you want a quicker approach, then you can cut the entire plant down to about 6″-8″ from the ground. This method is called rejuvenation pruning. The best time to do this is also in the early spring before the plant leafs out. In the latter case you will sacrifice all the blooms on your lilac shrubs; however you will be rewarded with a flush of new branches. In the case renewal pruning some of the blooms will also be sacrificed.
This article has more information on lilac pruning
Good luck with your lilacs.